The purpose of this project is to characterize the composition of mineral phases in cultured bone tissue grown in a hollow fiber bioreactor system. Unstained sections of plastic-embedded material were examined by electron microscopy. Polycrystalline ring electron-diffraction patterns from the mineralizing deposits within these specimens were clearly visible only after energy-filtering, which successfully removed the high background produced by inelastic scattering. By indexing these patterns it was possible to identify the deposits as hyroxyapatite. This composition was confirmed by recording x-ray emission spectra from the same regions of the specimens. The measured ratio of calcium to phosphorus was consistent with the presence of the apatitic mineral phase. Such measurements are relevant for assessing de novo bone formation in implants composed of bone-like ceramics.